With the spreading of digital broadcast or broadband, there has been a situation in which a large amount of digital contents have arrived at each home and information recording is required to have a further higher density. With regard to optical disc storage system, recording density has been developed from 700 MB CD (Compact Disc) using a light beam having a wavelength of 780 nm to 4.7 GB DVD (Digital Versatile Disc) using a light beam having a wavelength of 650 nm. Furthermore, in recent years an optical disc system having a capacity of more than 20 GB has been realized by using a light beam having a wavelength of 405 nm.
With such a high density recording system, since it is necessary to provide a compatibility to DVD which has been spread widely, it is necessary for an optical pickup to mount a laser device capable of emitting a light beam having a wavelength of 650 nm.
As to an optical pickup capable of emitting a plurality of light beams having different wavelengths, it is required to use a two-wavelength integrated laser device so as to make the optical pickup compact in size and light in weight. However, with regard to GaN based semiconductor which realizes a laser having a wavelength around 405 nm as well as AlGaInP based semiconductor which realizes a laser having a wavelength around 650 nm, since their physical properties differ greatly from each other, it is impossible to realize a monolithic integration on the same substrate. To solve this problem, there have been suggested an improved two-wavelength integrated laser device based on a hybrid structure (patent document 1: Japanese Unexamined Patent Applicant Publication No. 2001-230502; patent document 2: Japanese Unexamined Patent Applicant Publication No. 2000-252593).
A two-wavelength integrated laser device disclosed in patent document 1 is formed by stacking and attaching a first light emitting element and a second light emitting element on to a support substrate (so called sub-mount), thereby realizing a semiconductor laser device having a hybrid structure. Here, the first light emitting element has a first substrate and can emit a laser light having a short wavelength (for example, around 405 nm), while the second light emitting element has a second substrate and can emit a laser light having a long wavelength (for example, around 650 nm).
In more detail, the first light emitting element is attached to the support substrate in a manner such that its light emitting portion is positioned on the first substrate's one side facing the support substrate, while the second light emitting element is attached to the first light emitting element in a manner such that its light emitting portion is positioned on the second substrate's one side facing the first light emitting element.
A semiconductor laser device having a hybrid structure disclosed in patent document 2 is formed by electrically bonding n-electrode and p-electrode of a second laser section top-electrode and n-electrode of a first laser section through a fused metal so as to form an arrangement not containing the substrate of the first laser section, thereby making it possible to emit a plurality of laser lights having different wavelengths by virtue of the first laser section and the second laser section.